This invention relates in general to a front fork for suspending a front wheel of a motorcycle and, more particularly, to an improvement over the front fork of such a type in which an inner tube is slidably disposed in an outer tube.
Generally, in a front fork for suspending a front wheel of a motorcycle, it is required that a damping effect be freely adjusted to an optimum value at all times in dependence on the differences in the weight of the motorcycle and running conditions of the motorcycle such as, for example, in normal running or racing. Another requirement is that the amount of air-bladders or bubbles to be mixed with working oil be reduced as much as possible in order to stabilize the damping effect. Further, if the working oil is sucked into a lower chamber of the front fork having its volume increased during operation of the front fork in an insufficient amount, vacuum is produced in the lower chamber so that air contained in the working oil is set free therefrom and compressed and smashed at the subsequent stroke of the front fork thereby producing unusual sounds therein. These sounds cause the driver to experience a feeling of unrest and, hence, it is desirable to preclude such an undesirable phenomenon. Nevertheless, no attempts to meet these requirements have been made in the prior art front forks of the type in which the inner tube is slidably disposed in the outer tube, for various reasons. One of these reasons resides in the fact that an orifice for producing a damping effect is formed in a hollow rod extending from the bottom wall of the outer tube into the inner tube and, therefore, it is difficult to change the effective cross-sectional area of the orifice after the front fork has been completely assembled. Another reason is based on the fact that the orifice is necessarily formed in the hollow rod at a position near the air chamber provided in the inner tube due to inherent construction of the front fork by which working oil ejecting from the orifice is apt to be mixed with air in the air chamber. Among other reasons, when the lower chamber of the outer tube is increased in volume during the expanding stroke of the front fork, the working oil is sucked into the lower chamber only through the elongated hollow rod having a small diameter and, thus, it is difficult to prevent the lack of amount of working oil sucked into the lower chamber, due to the inherent construction of the front fork.